The invention relates to an electron tube comprising in an evacuated envelope a mesh or cage cathode and an anode.
Such electron tubes have a wide field of application. They are used, for example, as diodes, triodes, or tetrodes. These tubes may have a planar structure or may be constructed coaxially. Tubes of this type are used, for example, as rectifiers and as transmitter tubes for radio and television, and also as transmitter tubes for heating purposes.
Such a tube, in particular a transmitter tube, is known from the book "Tubes for RF-heating" by H. F. Dittrich, Publications Dept. of Philips' Electronic Components and Materials Division, Eindhoven, October, 1971, which may be considered to be incorporated herein by refernece. A number of systems are described in the book (see pages 118-120) for the control of the output power of transmitter tubes. None of these systems is simple. Moreover, the systems often lead to considerable power losses. These tubes comprise a mesh or cage cathode. The grid used in these tubes also usually has a mesh or cage structure. A mesh cathode usually consists of two sets of crossing parallel wires which are welded together at the crossings. These wires usually consist of carbonized thoriated tungsten. A cage cathode consists of two sets of parallel wires crossing each other at an angle of 90.degree.. One set of wires in such a cage cathode extends parallel to the cathode axis and is situated on a cylindrical surface. Cage cathodes are also known in which a set of wires extends parallel to the cathode axis and is situated on a cylindrical surface and one or more coils are wound around the set of wires. However, such a cathode may also be manufactured from a foil cylinder of, for example, carbonized thoriated tungsten sheet having diamondshaped, square, triangular or elongate apertures, so that a mesh or cage cathode is also obtained.
German Patent Application No. 1,639,404 laid open to public inspection discloses a transmitter tube having around an axis a tubular anode in which a cathode and a control grid are present coaxially. In the cathode a focussing electrode is accommodated which has a number of grooves extending parallel to the axis in which stripshaped cathode parts extend parallel to the axis. Flat electron beams are formed by this structure which are directed outwards radially.